Synaptic Transmission from Horizontal Cells to Cones Is Impaired by Loss of Connexin Hemichannels

Abstract

In the vertebrate retina, horizontal cells generate the inhibitory surround of bipolar cells, an essential step in contrast enhancement. For the last decades, the mechanism involved in this inhibitory synaptic pathway has been a major controversy in retinal research. One hypothesis suggests that connexin hemichannels mediate this negative feedback signal; another suggests that feedback is mediated by protons. Mutant zebrafish were generated that lack connexin 55.5 hemichannels in horizontal cells. Whole cell voltage clamp recordings were made from isolated horizontal cells and cones in flat mount retinas. Light-induced feedback from horizontal cells to cones was reduced in mutants. A reduction of feedback was also found when horizontal cells were pharmacologically hyperpolarized but was absent when they were pharmacologically depolarized. Hemichannel currents in isolated horizontal cells showed a similar behavior. The hyperpolarization-induced hemichannel current was strongly reduced in the mutants while the depolarization-induced hemichannel current was not. Intracellular recordings were made from horizontal cells. Consistent with impaired feedback in the mutant, spectral opponent responses in horizontal cells were diminished in these animals. A behavioral assay revealed a lower contrast-sensitivity, illustrating the role of the horizontal cell to cone feedback pathway in contrast enhancement. Model simulations showed that the observed modifications of feedback can be accounted for by an ephaptic mechanism. A model for feedback, in which the number of connexin hemichannels is reduced to about 40%, fully predicts the specific asymmetric modification of feedback. To our knowledge, this is the first successful genetic interference in the feedback pathway from horizontal cells to cones. It provides direct evidence for an unconventional role of connexin hemichannels in the inhibitory synapse between horizontal cells and cones. This is an important step in resolving a long-standing debate about the unusual form of (ephaptic) synaptic transmission between horizontal cells and cones in the vertebrate retina.

MRI of rod cell compartment-specific function in disease and treatment in vivo

“…A recent study in zebrafish suggested the controversial hypothesis that HC-specific connexin 57 (Cx57) regulates inhibitory feedback to photoreceptors (Ciolofan et al, 2007;Dedek et al, 2008;Hombach et al, 2004;Kamermans et al, 2001;Klaassen et al, 2011;O'Brien et al, 2012a,b;Pandarinath et al, 2010;Shelley et al, 2006). Connexins, transmembrane proteins that form channels that make up hexameric hemichannels between apposed cells, provide a communication path between neurons and contribute to information processing.…”

Connexin and pannexin signaling pathways, an architectural blueprint for CNS physiology and pathology?

“…In fish retina, HCs and Panx channels have been shown to contribute to a very fast ephaptic and a relatively slow proton-mediated mechanism, respectively (see Fig. 5b for more detailed information concerning the signaling mechanisms) [158][159][160][161][162]. HC/ Panx1 channel-mediated ephaptic transmission was also recently demonstrated in the mouse retina, albeit it is not completely clear whether it involves HCs and/or Panx1 channels [30,163].…”

“…Ephaptic transmission relies on an alteration in the excitability of cones due to electric fields that are generated by a HorC proximal to the cone. HCs on horizontal dendrites open with HorC hyperpolarization and allow inward current flow [158]. When the retina receives a light stimulus in the surround receptive field, the hyperpolarization of HorCs`increases the inward current flow through the HCs, resulting in an increased negativity of the synaptic cleft.…”

Constructing the rod bipolar cell signalplex using animal models of retinal dysfunction.

“…The horizontal cells form inhibitory contacts with rod spherules and cone pedicles and are electrically coupled to one another by gap junctions (Kolb, 1970;Klaassen et al, ! !…”